1. Field of the Invention
The invention relates to a voltage generator device and a motor vehicle that supply an increased drive voltage to an electric load at the time of reduced power supply voltage, a control method for the voltage generator device, a control method for the motor vehicle, and a computer-readable recording medium storing a program for causing a computer to execute such a control method.
2. Description of the Related Art
Idle-stop vehicles that automatically stop the engine in a temporary fashion during a vehicle stop for a traffic signal or the like have been commercialized in recent years in order to reduce the fuel consumption and the exhaust emissions.
In such an idle-stop vehicle, the engine is automatically stopped provided that a condition for stopping the engine is met and that the vehicle is considered to be at a stop. Subsequently, when a condition for starting the engine is met, the engine is automatically started by a starter in order to get the idle-stop vehicle under way.
The starting of the engine by the starter requires a considerable amount of electric power. Therefore, the battery that serves as an electric power supply in the idle-stop vehicle is heavily discharged if the engine is frequently restarted, for example, during city street driving where frequent stops-and-goes are experienced.
If the engine is started from a stopped state when the battery is significantly discharged, the voltage of the battery temporarily falls to a great extent as the starter consumes electric power. The electric loads connected to the battery include electric loads having a CPU (central processing unit), for example, a car audio device, a car navigation system, etc. If the battery voltage falls below the reset voltage of a CPU (e.g., the power supply voltage of 8V of an electric load), there is possibility that the CPU may be reset and various settings retained during the running of the vehicle may be reset.
In order to resolve such troubles regarding electric loads, Japanese Patent Application Laid-Open Publication No. 2002-38984 discloses a related-art technology in which if the battery voltage reduces at the time of restart of the engine after an idle stop, the battery voltage is raised from the reduced level before being supplied to an electric load.
In a conventional arrangement incorporating a DC/DC converter or the like as a voltage generator device, operation of the voltage generator device is stopped for the purpose of circuit protection if an overvoltage or overcurrent exceeding a target value of voltage or current is detected. The stop of operation of this device is immediately followed by a sharp reduction of the output voltage supplied to the electric load, so that the following problem occurs, that is, the voltage compensation cannot be attained and the driving of the electric load cannot be appropriately controlled.
FIG. 10 is a waveform diagram for describing the output during overcurrent in a related-art technology. With reference to FIG. 10, an example case where a current value of 30 A or greater is set as an overcurrent will be described. Until the output current reaches 30 A, the output voltage remains at a constant voltage (e.g., 12 V) for compensating for the battery voltage fall. As the output current increases, the value of voltage of an overcurrent detection output for detecting an overcurrent state of the output current increases. If this value of voltage exceeds an overcurrent detection threshold value, the operation of the voltage generator device is stopped.
FIG. 11 is a waveform diagram for describing a control performed during overvoltage in the related-art technology. With reference to FIG. 11, let it assumed that the output voltage exceeds, for some cause, a target voltage (e.g., 12 V) of a voltage raising operation, and reaches an overvoltage prohibition threshold value (e.g., 16 V) that is close to the withstanding voltage of the electric load. In that case, an overvoltage stop output is output so as to stop the operation of the voltage generator device. Therefore, the input voltage is directly output. With regard to FIG. 11, it is to be noted that the output voltage is slightly lower than the input voltage due to the internal resistance of the voltage generator device.
Thus, since an ordinary voltage generator device, such as a DC/DC converter or the like, is designed taking a long-time continuous operation into consideration, the operation thereof is generally stopped, for example, for protection of an element or device, if an abnormality is detected. If such a control is applied to a voltage generator device that compensates for a voltage fall at the time of start of the engine after an idle stop, the problem of being unable to appropriately control the driving of a necessary electric load occurs also when the engine of a vehicle having a trouble is started in order to transport the vehicle to a repair shop.
Document EP 0 483 744 A2 discloses a current detection circuit of a power semiconductor device and a power converter using the circuit, wherein different detection levels of a load current can be detected. These detection levels are an overcurrent control level and/or a constant current control level. Said constant current control level can be a constant current value associated with chopper control or a value which judges an overload when a chopper control current exceeds a predetermined current level beyond a predetermined number of times. Said overcurrent control level can be a value corresponding to short-circuit-protection.
Further, a control circuit controls power semiconductor devices by decreasing a control voltage in a plurality of steps to zero in the case where an overcurrent detecting circuit outputs a detection signal corresponding to an overcurrent control level.
Further, document EP 1 079 496 A2 discloses an abnoramlity detection apparatus for a power supply circuit associated with an internal combustion engine that detects an abnormality that may occur in a power supply circuit, and controls the automatic stop and the automatic start of the engine based on the state of the power supply circuit. The state of charge/discharge of a battery determined based on the electric potential of a terminal located between the battery and a generator-motor or a load is compared with the current through the battery detected by an ammeter. If there is a contradiction therebetween, it is determined that an abnormality has occurred somewhere in the power supply circuit including the battery. When it is determined that the power supply circuit has an abnormality, the apparatus performs such a control as to prevent the automatic stop/start control in which the engine is automatically stopped if a predetermined condition is met, and in which the engine is automatically restarted when the condition is unmet after being met.
Still further document U.S. Pat. No. 3,784,893 discloses a high voltage shutdown protection circuit with bias arrangement to decrease the voltage shutdown point with increasing load. There, a current and voltage regulated DC to DC converter is designed with protection features to permit its operation in parallel with like DC to DC converters. The features are designed to assure shutdown protection against overcurrent and overvoltage conditions and to assure that each converter will contribute a current to the common load. The protection features include a selective high voltage shutdown to shut down only the converter causing an overvoltage and an overcurrent protection circuit to supersede the normal current regulation in response to a fault condition. A reverse current shutdown circuit protects the common load from faults and short circuits internal to the converter circuit. These protection features permit the converters to operate in parallel without shutting down the entire system should an individual converter malfunction.